You see inflated results for the 'sum' because it also sums the suppressed records.
If you're OK with displaying teh summary on the Group Footer, you can create a Running Total field. Go to the Field Explorer > RIght-click Running Total fields > Move the @resolutiontime formula to the 'Field to summarize' > Type of summary should be sum > Under Evaluate select the formula field beside 'use a formula' and use this code:
{CallLog.CallID} <> Next( {CallLog.CallID})
Then, under Reset select 'on change of group' and select the Group #1.
If you're not OK with displaying the summary on the footer, you have two choices:
1) Use a Subreport that returns the sum - expensive option
2) Use a SQL Expression that finds the first date for each group and use that in the Record Selection Formula to so that you only one row per group. You can then simply insert a summary on the resolutiontime formula and show it on the GH.
E.x SQL Expression:
(
Select Min("Received_Date") from Table
where "GroupName" = "Assignment.GroupName"
)
Record selection formula:
<existing selection formula>
AND
{Callog.Received_Date} = {%SQL Expression}
3) Create a SQL Query that returns the fields you need.
-Abhilash